1. Field of the Invention
The present invention relates to a method and device for improving audio reproduction in magnetic tape recording particularly the bass response wherein the low frequency band of the audio signal is recorded on an AM carrier with the frequency of the carrier at least 19 kHz and recorded on the same track as the rest of the audio signal which is directly recorded onto the tape. During playback, the carrier is recovered using a highpass or bandpass filter, demodulated and the low frequency signals obtained are mixed with the directly recorded audio.
2. Description of the Prior Art
The method of using an ultrasonic carrier for recording information on audio tape with audio frequency information also directly recorded onto the same track is known in the art. U.S. Pat. No. 4,333,113 discloses a method of recording a coded digital signal on an FSK (frequency shift keying) high frequency (beyond 15 kHz) carrier for monitoring the reproduction of recorded information. Russian patent SU1269185 (November 1986) discloses a method of recording the low frequency component of an analog signal onto an FM carrier in the frequency range above the highest frequency component of the analog signal to reduce cross talk in multitrack recording. German patent DL 0213777 (September 1984) discloses a method wherein the frequency spectrum of an analog signal is inverted using a ring modulator and a 20 kHz carrier and recorded onto magnetic tape. The low frequency components of the analog signal are converted to frequency components just below 20 kHz and a 19 kHz component, for example, of the analog signal is converted to 1 kHz in the inverted signal. During playback, the carrier is regenerated and the signal is reinverted using a ring modulator to obtain the original signal. This process also is designed to reduce cross talk in multitrack recording.
The disadvantage with the Russian process is that recording analog signals onto an FM carrier requires very strict requirements on wow and flutter in the tape transport mechanism. This is especially true when recording narrow band FM. Wow and flutter is equivalent to FM as far as the carrier is converned. According to Selsted et al., using R.D.B. standards the usual deviation is 71/2%. To get an accuracy of 1%, the speed variations of the recorder would have to be limited to about 0.1% peak-to-peak. Assuming the flutter is sinusoidal this would correspond to 0.035% of the usual rms flutter-measuring standard. This would require a high precision transport mechanism.
Specifications for wow and flutter of typical home cassette decks on the market can range from about 0.06% for very high quality cassette decks to about 0.15% for average cassette decks.
Another problem with the Russian process is that changes in tape speed can cause the low frequency components on the FM carrier and the directly recorded audio to be shifted in pitch by unequal amounts. If changes in tape speed occur at a rate comparable to the modulating frequencies in the FM signal this can cause phase shifting in the modulating frequencies. Since phase shifting or phase modulation is related to frequency modulation the modulating frequencies can themselves be frequency modulated by flutter. Much slower changes in tape speed (i.e. wow) produce an amplitude modulating effect on the modulating frequencies. These effects would tend to produce a kind of out of tune effect when FM and directly recorded audio were multiplexed onto the same tape track. A large enough deviation of the FM carrier (i.e. .+-.30% ) could be used to reduce the error to an acceptable level but this would require a much greater bandwidth (i.e. .+-.30% deviation of a 19 kHz carrier would have a bandwidth of at about 11.4 kHz)
Disadvantages also exist with the German process. Inverting the audio requires that the whole audio range be processed (i.e. inverted) requiring low distortion processing to be comparable to direct recording. Another problem with inverted audio is that if changes in the frequency or amplitude of a high frequency component occurs faster than the period of the corresponding lower frequency component of the inverted audio signal distortion results. For example, a single wavelength of a 19 kHz signal (0.0526 ms period) could not be converted to a 1 kHz signal (1 ms period) without introducing distortion.